Introduction

Cover crops play an important role in maintaining soil quality
and productivity on Ontario farms. Usually we think of cover crops
in terms of reducing soil erosion and adding organic matter to the
soil  but they can do much more:

Cover Crop Functions

Reduce Soil Erosion

Cover crops can do exactly what their name implies; cover the soil.
A cover crop such as rye
is commonly used to cover and protect the soil surface from wind
and water erosion. The top growth covers the soil surface while
the roots bind and stabilize the soil particles.Cover crops may
be planted over a whole field for erosion protection, or they may
be selectively planted in the most erosion prone areas.  such
as sandy knolls for wind erosion, water runs or low areas for water
erosion.

Figure 1 - Barley interseed

Figure 2 - Cover crop of rye planted
in high sand knolls

Good stand establishment and vigorous growth
is important to ensure adequate erosion protection. For the most erosion
prone areas, this can be a real challenge. For example - in a field
with high sand knolls often it is difficult to get a good growthy
cover crop on the knolls. The lack of growth/stand can be due to a
low organic matter, pH problems, droughtiness etc. This is also the
most erosion prone area and may "blow out" over winter if
there is little snow cover due to the lack of cover crop protection.
Its essential to plant into moisture.

Add Organic Matter

Cover crops do add organic matter but the amount really varies
depending upon the cover crop species and the conditions under which
it is grown. The table below gives dry matter levels(above ground
residues only) for some common cover crops established in late summer
and early fall. The numbers were collected from cover crop plots
in Southwestern Ontario.

Common Cover Crops1

Dry Matter Produced kg/ha

Red Clover (plowdown)

2,700 - 4,500

Oats

1,000 - 5,500

Rye

1,000 - 4,000

Oilseed Radish

2,000 - 7,500

1Dry matter production can vary greatly
due to a number of production factors

These may not show as great an organic matter return as some of
the cover crop work from the United States, however it is important
to consider climatic differences and how the cover crop is incorporated
into the crop rotation.

It would take 20 years to build the organic matter 1% (provided
that the soil was never worked to speed up decomposition). But don't
despair! It may be a slow process, but it's possible to improve
over time. Cover crops and manure certainly help.

Work to either improve or at least maintain organic matter.
If you do nothing and continue cropping, your organic matter levels
will continue to drop.

Building organic matter is a slow process, maintaining it is probably
more realistic. Current research suggests that only 20 % of any
residue returned to the soil will make it to the organic matter
pool. The remaining 80 % becomes part of living organisms, is released
as gases during digestion or has not become part of the organic
matter flow.

For example: Soil organic matter is measured in the top
15 centimeters or to plow depth. This "hectare/furrow slice"
weighs about 2,000,000 kilograms. Thus, 1% organic matter equates
to 20,000 kilograms.

In a best case scenario, only 20% of any residue returned to the
soil will make it to the organic matter pool. The remaining 80%
becomes part of living organisms, is released as gases during digestion,
or has not become part of the organic matter flow.

It takes 5 kilograms of crop residues to make 1 kilogram of organic
matter.

Thus, it requires 100,000 kilograms of crop residue to raise the
soil organic matter 1%. Assuming an average residue return of 5,000
kilograms from the above table:

100,000 kg residue ÷ 5,000 kg residue/yr = 20 years

The addition of fresh organic material is important in stimulating
soil biological activity. Cover crops also help to maintain soil
life by shading the soil and moderating soil conditions.

Reduce Nutrient Losses

Some cover crops fix nitrogen but many more require nitrogen to
grow. Grass cover crops like rye
and Brassicas like oilseed
radish are excellent scavengers of nitrogen left behind by the
main crop or from manure applications.

Cover crops that take up nitrogen can help to reduce nitrogen losses
due to leaching. This reduces the potential for the contamination
of shallow aquifers by nitrates.

Nitrogen " catch crops " are used to prevent winter
injury to perennial crops such as grapes and fruit trees. Cover
crops such as rye seeded in late summer and early fall take up the
excess nitrogen  preventing the perennial crop from continuing
vegetative growth and encouraging hardening off for the winter.

When the cover crop is killed the nutrients held in the plant tissues
are returned to the soil and can be used by the following crop.

Figure 3 - Cover crop of oilseed
radish

Work done by in Ontario Dr. Eric Beauchamp on cover crops and nitrogen
release suggests that growers should be aware of the nitrogen release
potential from their cover crop species. Oilseed radish
is an excellent nitrogen scavenger but as it winterkills the tissues
start to break down before the spring crop is in place and needing
nitrogen. Some nitrogen losses can occur. In contrast, ryegrass
tends to hold onto the nitrogen more tightly, with the main release
occurring in early to mid summer. Clovers
tend to release nitrogen in early summer also. Timing of the nitrogen
release may be important if growing crops like small grains, tomatoes
and other fruiting vegetables.

Improve Soil Fertility

Legume cover crops can fix nitrogen for a subsequent crop. Organic
growers often include this type of cover crop in order to produce
nitrogen for the rest of the crop rotation. Some cover crop species
are credited with making more phosphorous available to other crops
through the action of the roots. Deep-rooted cover crops can bring
nutrients up from deep in the soil profile

Figure 4 - Alfalfa root systems

Reduce Pest Populations

Some cover crop species maybe a non-host for a pest
or may release materials that are toxic to the targeted pest.For
example many common cover crops have been rated for their ability
to support root lesion and other nematode populations (see Chart)
Cover crops like marigolds
and pearl
millet do not support or do not allow the nematode to reproduce.
Other cover crops like some mustards,
particularly those with high glucosinilate and euricic acid levels
in the plant tissue can create a "natural fumigant" through
the chemical breakdown of the these materials. The amount of green
plant material that must be tilled into the soil for this to be
effective, is often difficult to achieve under field conditions.Weeds
are often alternate hosts for the nematodes. Good cover crop establishment,
adequate plant stand and excellent weed control within the cover
crop is critical for these methods of nematode reduction/suppression
to be effective.

Soybean cyst nematode
(SCN) causes the most economically important disease of soybeans
in North America. The estimated annual loss in southwestern Ontario
is $5 to 10 million. Research done at Harrow (AAFC) has indicated
that rotational crops of red clover, Japanese millet, field corn
and resistant soybeans have the greatest effect in reducing populations
of SCN. Perennial ryegrass, alfalfa, sorghum, and hairy vetch are
not as effective in reducing SCN populations. Unfortunately, most
of these nematode reducing crops are not profitable enough to fit
into most growers rotations.

Further research is ongoing to develop crop rotations that are
cost-effective and able to reduce the SCN pressure. Currently winter
wheat (a non-host) with an underseeded legume crop (stimulates the
SCN to hatch) is the favoured approach. Legumes for wheat underseeding
that are most suited to encourage hatching are being evaluated.
A similar approach is planned for underseeding in corn.

Figure 6 - Adult female root-lesion
nematode (150x magnification)

Figure 7 - Rye Cover Crops used as
wind strips

Research work in Ontario on rye cover crops used as wind strips
and in no-till systems for processing tomato production has found
that rye will act as a physical barrier to pests. Dr. David Hunt
and Dr. Al Hamill of AAFC in Harrow have documented that rye cover
crops interfere with Colorado potato beetle movement early in the
spring, during tomato transplanting.

Cover crops can be helpful in controlling or suppressing pests,
however the opposite is true. It is important to consider pest relations
when choosing
a cover crop

Figure 8 - Firefly

Cover crops can also provide habitat for bees and
beneficial organisms. Predatory mites, ladybug beetles and other
beneficial insects benefit from the cover and protection that a
cover crop provides.

Reduce Compaction and Improve
Soil Structure

Cover crops can help to reduce compaction and improve soil structure.
The addition of the plant top and, especially root matter, helps
to improve water infiltration and holding ability. It can also decrease
soil bulk density. Deep rooted cover crops can help to decrease
the impact of soil compaction. If deep tillage is used to combat
compaction, the seeding of a cover crop can help to stabilize the
cracks and fissures that were created in the soil profile by the
deep tillage.Growing cover crops and working in the residues improves
soil structure.

Figure 10 - Cover crop in a tomato
rotation

In the early '90's a tomato rotation and cover crop project in southwestern
Ontario (ref) found that there was a sizable benefit to the return
of almost any crop material to the soil. This was particularly true
of one site with an extremely low organic matter fine sandy loam.
Soil crusting and lack of aeration was typically a problem unless
residues were incorporated. One of the best yielding rotation was
an intensive vegetable and seed corn rotation that allowed the growth
and incorporation of a sizable amount of cover crop residues.

Water Management

Cover crops can be grown and the residues used to mulch the crop
and help to reduce moisture loss.

In contrast, some growers use cover crops such as rye
as a way to reduce soil moisture in early spring. The crop takes
up the soil moisture and uses it within the plant. The water is
incorporated in the plant tissues or is transpired. Either way the
water is removed from the soil.

Caution : this use of cover crops requires very
good management and understanding of plant growth habits. Under
warm wet conditions some cover crops can become very difficult to
control either with tillage or chemically. Also under dry spring
conditions the cover crop should be controlled earlier to prevent
excessive moisture loss which may interfere with crop establishment
and growth.

Figure 11 - Even a small amount
of crop residue can provide a benefit to a growing crop under extremely
dry conditions. The residue moderates soil temperatures and help
to reduce evaporation of soil water.

Emergency Forage

Under adverse conditions and a shortage of forage due to drought
or winterkill, some cover crop species can make quite acceptable
hay or pasture.

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